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Old   March 8, 2015, 09:05
Default Error in expression unit
  #1
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Hi,
To create velocity profile at inlet I have used the following expression

15[m/s]*(z/0.3)^(1/7)
where 15 is my reference velocity, 0.3 is my reference height and 0.143 is power law exponent and z is height at any point in z direction

after plotting I am getting the exact values which I require but with invalid units.
the error is as follows:

Expression resolves to invalid units ('m^0.143 s^-1') in value set for parameter 'Normal Speed' in object '/FLOW:Flow Analysis 1/DOMAINomain 1/BOUNDARY:inlet/BOUNDARY CONDITIONS/MASS AND MOMENTUM'. Expected units: 'm s^-1'.

I am unable to resolve this error. Please shed some light as to what needs to be added or rectified.

CCl for reference:

# State file created: 2015/03/08 17:16:04
# CFX-14.5 build 2012.09.19-21.47

FLOW: Flow Analysis 1
DOMAIN: Domain 1
BOUNDARY: inlet
Boundary Type = INLET
Location = INLET
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
MASS AND MOMENTUM:
Normal Speed = 15[m/s]*(z/0.3)^(1/7)
Option = Normal Speed
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
END
END
END
COMMAND FILE:
Version = 14.5
END
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Old   March 8, 2015, 18:21
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Glenn Horrocks
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You just cancel out the length dimension before you do the exponent:

15[m/s]*(z/0.3[m])^(1/7)

This happened because you forgot that your reference length has the units of length (presumably metres). If you included the dimension in your expression it works fine.
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Old   March 10, 2015, 06:16
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Hello,
Thank you for responding to my query. I tried the correction you suggested and the expression works fine.

However in solver I am getting an error because of the expression. The error is as follows:

PROBLEM ENCOUNTERED WHEN EXECUTING CFX EXPRESSION LANGUAGE |
| |
| The CFX expression language was evaluating: |
| Normal Speed |
| |
| The problem was: |
| INVALID-EXPONENTIATION |
| |
| FURTHER INFORMATION |
| |
| The problem was encountered in executing the expression for: |
| $SCALAR_0 |
| The complete expression is: |
| 15[m/s]*(z/0.3[m])^0.143 |
| The error occurs on sub-expression: |
| (z/0.3[m])^0.143 |
| |
| BACKGROUND INFORMATION |
| |
| The error was detected at one location. The same problem may be |
| present at other locations - that has not been investigated. |
| The following values are for the first location which has the |
| problem. |
| |
| In this expression, these sub-expressions have been calculated: |
| |
| Sub-expression = Value |
| |
| 15[m/s] = 15.0000 |
| These values were set before reaching the current expression: |
| |
| Name = Expression = Value |
| |
| z = $LOCAL_z = -6.245005E-17 |
| |
| END OF DIAGNOSTIC OUTPUT FOR CFX EXPRESSION LANGUAGE

I am unable to interpret what exactly is wrong in the subexpression.
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Old   March 10, 2015, 06:24
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when z is negative the result of your expression (z/0.3[m])^0.143 becomes imaginary, which CFX cannot handle. If you only have positive Z values in your domain I would add an abs() to z.
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Old   March 11, 2015, 05:25
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Hello,

Thank you for explaining about the reasoning behind the error. I tried the expression by including abs(z) and now i am not getting any errors in solver.

Thanks and Regards

Banu
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Old   July 14, 2016, 05:37
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Hey guys,

I had a similar error:

ERROR PROBLEM ENCOUNTERED WHEN EXECUTING CFX EXPRESSION LANGUAGE The CFX expression language was evaluating: Input from CFX-5 The problem was: INVALID-EXPONENTIATION FURTHER INFORMATION The problem was encountered in executing the expression for: HSOut The complete expression is: HIn+cpRef*tIn*((pOut/pIn)^((gamma-1)/gamma)-1) The error occurs on sub-expression: (pOut/pIn)^((gamma-1)/gamma) BACKGROUND INFORMATION In this expression, these sub-expressions have been calculated: Sub-expression = Value cpRef*tIn = 165221. These values were set before reaching the current expression: Name = Expression = Value tIn = massFlowAveAbs(Temperature)= 164.497 pOut = 1 [atm] + massFlowAveAbs(Pr= -50895.4 pIn = 1 [atm] + massFlowAveAbs(Pr= 117734. gamma = 1.40000 cpRef = massFlowAveAbs(Specific Hea= 1004.40 HIn = massFlowAveAbs(Static Entha= -134241. END OF DIAGNOSTIC OUTPUT FOR CFX EXPRESSION LANGUAGE

Any thoughts?

Brett
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